Ocean Drilling Program Scientific Results Volume

Home , Poikilitic

Von Herzen, R. P., Robinson, P. T., et al., 1991 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 118

22. PLASTIC DEFORMATION AT AN OCEANIC SPREADING RIDGE: A MICROSTRUCTURAL STUDY OF THE SITE 735 GABBROS (SOUTHWEST INDIAN OCEAN)1

Mathilde Cannat2

ABSTRACT Microstructural analysis suggests that some Site 735 gabbros were deformed in the crystal mush stage, before they had completely crystallized. Later, solid-state deformation began in granulite-facies metamorphic conditions. At this stage, temperature-dependent diffusion processes may have controlled the plastic deformation of these gabbros, and mylonites formed only in intervals containing more than 10% Fe-Ti oxides. Most oxides in these mylonites are interpreted as magmatic in origin. A sharp change of deformation processes occurred as temperature decreased to the stability conditions of amphibolite-facies metamorphic assemblages: dislocation slip in plagioclase may have become the leading plastic flow mechanism. This change of deformation processes coincides with increased availability of hydrothermal water and with a marked decrease of the plagioclase recrystallized grain size. This reduction in recrystallized grain size is thought to result from an increase of the deviatoric stress, and thus from an increase of the yield strength of the gabbros. In amphibolite-facies metamorphic conditions, dynamic recrystallization of plagioclase locally caused strain-softening, leading to the formation of mylonites. Relict porphyroclasts (plagioclase, olivine, and pyroxenes) in these mylonites are fractured and boudinaged. The cracks produced by this brittle failure are filled with green to brown hydrothermal hornblende.

INTRODUCTION the Site 735 deformed gabbros. These results are used to interpret the tectonic evolution of these gabbros (Cannat et A total of 500 m of gabbro was drilled at Site 735 during al., this volume). These results should also be relevant to Ocean Drilling Program (ODP) Leg 118. Site 735 lies on a more general studies of rheological behavior of the oceanic shallow platform near the east wall of the Atlantis II Fracture lithosphere near a spreading ridge axis. Zone, about 100 km south of the Southwest Indian Ridge axis Most Site 735 gabbros contain more than 50% plagioclase. (Fig. 1). The pattern of magnetic anomalies indicates that the This mineral thus may be expected to have controlling influ- crust here was accreted at the ridge about 11 m.y. ago (Dick et ence during plastic deformation. In the following microstruc- al., this volume). This is supported by a zircon isotopic age of tural analysis, I chose plagioclase as the key mineral for 11.3 Ma (J. Mattinson, pers. comm., 1989). textural and preferred orientation research. Among the mafic About 30% of the 435 m of gabbro recovered at Site 735 is minerals, I emphasize olivine because its dislocation substruc- plastically deformed along flat to moderately dipping, normal ture can be observed readily under an optical microscope, shear zones (Cannat et al., this volume). The synkinematic using the oxidation technique described by Kohlstedt et al. assemblages in these shear zones correspond to metamorphic (1976). The anorthite (An) content of Site 735 magmatic conditions that range from granulite to lower amphibolite plagioclase varies between 25% and 80%. Because the rheo- facies (Stakes et al., this volume). There is no evidence in the logical properties of plagioclase vary with its An content hole for significant ductile deformation below the stability (Gandais and Willaime, 1984), all fabric measurements were temperature of actinolitic hornblende, and the offset accom- performed on samples having similar plagioclase An contents modated by brittle failure of the gabbros is limited. Deforma- (An35 to An50). However, textural observations of samples tion at Site 735 thus occurred when the gabbros were still hot, having a different An content suggest that composition did not probably in the vicinity of the Southwest Indian Ridge axis control the modes of plastic deformation of plagioclase at Site (Cannat et al., this volume). 735. In samples deformed in the stability conditions of gran- The distribution of the synkinematic assemblages in the Site ulite facies or hornblende-bearing amphibolite-facies meta- 735 deformed gabbros indicates that this deformation occurred at morphic assemblages, the An content of the dynamically progressively decreasing temperatures (Stakes et al., this vol- recrystallized plagioclase is the same as that of the neighbor- ume), as may be expected in an oceanic domain, where newly ing magmatic porphyroclasts (Stakes et al., this volume). accreted lithosphere spreads away from a ridge axis thermal There is thus no composition-related rheological contrast to high. Water-rich hydrothermal fluids circulated in the gabbros be expected between magmatic and recrystallized plagioclase. during this plastic deformation, and in many intervals, the However, in samples deformed at temperatures correspond- temperature decrease during deformation correlates with an ing to the stability of actinolitic hornblende, plagioclase neo- increase in the water/rock ratio (Stakes et al., this volume). blasts are distinctly less calcic than magmatic porphyroclasts Here, I use the micro structures and the mineral preferred and than neoblasts produced at earlier, higher temperature orientations associated with ductile deformation to infer flow stages of the deformation (Stakes et al., this volume). In this mechanisms and the relative intensity of deviatoric stresses in paper, I do not investigate the possible rheological changes induced by this compositional variation. I concentrate on gabbros deformed either in granulite-facies or in hornblende- bearing amphibolite-facies metamorphic conditions. 1 Von Herzen, R. P., Robinson, P. T., et al., 1991. Proc. ODP, Sci. Results, 118: College Station, TX (Ocean Drilling Program). Some Site 735 gabbros display a foliation marked by the GDR "Génése et Evolution des Domaines Océaniques," UBO, 6 Av. Le planar-shaped fabric of the plagioclase, clinopyroxene, and Gorgeu, 29287 Brest Cedex, France. olivine crystals, with little or no recrystallization of these

399 M. CANNAT

for synkinematic circulation of hydrothermal water-rich fluids (Stakes et al., this volume). By contrast, the finer-grained recrystallized gabbros exhibit upper to lower amphibolite- facies metamorphic assemblages and ample evidence for synkinematic circulation of water-rich hydrothermal fluids (Stakes et al., this volume). Experimental evidence from metals, ceramics, and some minerals (Bird et al., 1969; Twiss, 1977; Mercier et al., 1977) indicates that, during steady-state plastic flow, the size of neoblasts produced by dynamic recrystallization decreases as the applied deviatoric stress RIDGE increases, regardless of temperature or finite strain. The recrystallized grain size in Site 735 gabbros thus probably reflects the relative intensity of deviatoric stress during plastic 32°OO'S deformation. A decrease in the size of recrystallized plagioclase in gabbros deformed at lower temperatures is consistent with an increase of the ductile strength expected in rocks when temperature decreases (see Kirby, 1985). The stress/ recrystallized grain size relationship in plagioclase has not been experimentally calibrated. Twiss (1977) proposed an "approximate theoretical relation," based on the equation of the dislocation strain energy in the grain boundary to that in the enclosed volume. Applied to the recrystallized grain sizes measured in the Site 735 gabbros, this relationship yields deviatoric stresses above 1.5 kb for the smallest neoblasts (5 to 10 µm), and below 1 kb for the largest ones (20 to 40 µm). These values are only indicative because (1) experimental Site 735 calibration is not available and (2) the recrystallized grain size in many samples is not unique and probably resulted from two distinct recrystallization mechanisms (rotation recrystalliza- TO tion and migration recrystallization). 33°00'S Microstructural study of the solid-state deformation in Site 735 gabbros has been organized around three questions: (1) what are the active flow mechanisms in the deformed gabbros having the largest recrystallized grain size (granulite-facies metamorphic conditions); (2) what are the active flow mechanisms in the deformed gabbros having smaller recrystallized -•-5011171 grain sizes (amphibolite-facies metamorphic conditions); and (3) what are the strain-softening mechanisms that enabled the strain to concentrate in narrow mylonitic shear zones? VISCOUS FLOW IN PROGRESSIVELY RIDGE SOLIDIFYING GABBROIC MAGMA Thick intervals of gabbro from the middle part of the hole (Cores 118-735B-36Rto 118-735B-56R; 172-272 mbsf; Cannat et al., this volume) display a foliation marked by tablet-shaped clinopyroxene and olivine crystals in a matrix of elliptic anhedral to euhedral plagioclase (Fig. 2A). In some samples, the preferred orientation of the long axis of these tablets 57°OO'E defines a faint lineation. Figure 1. Bathymetric map of the Atlantis II transform fault, showing Despite their strong shape fabric, these minerals have not the location of Site 735. Contour intervals at 1000 m (Survey from been intensely deformed. The clinopyroxene is neither kinked Conrad cruise 27-09, 1986, H. Dick, Chief Scientist, with D. Gallo and nor recrystallized. The olivine has not been recrystallized R. Tyce). either, but many grains exhibit dislocations (observed in oxidized samples; technique described by Kohlstedt et al., 1976), indicating some degree of plastic deformation. How- minerals (Fig. 2A; Fig. 3). This texture is interpreted here as ever, the crystallographic fabric of olivine is very weak: there resulting from laminar viscous flow of the progressively solid- is a poor concentration of [010] perpendicular to the foliation, ifying gabbros. but [100] and [001] are scattered (Fig. 5B). The plagioclase The solid-state plastic deformation textures in Site 735 often exhibits bent mechanical twins and lobate grain bound- gabbros range from weakly recrystallized and unfoliated (Fig. aries that suggest grain-boundary migration, but its recrystal- 4A), to extensively recrystallized and mylonitic (Fig. 4B). lization is limited (Fig. 2A). However, its crystallographic These textural differences are thought to reflect variable finite fabric is strong (Fig. 5A). [010] is preferentially perpendicular strain intensities. In addition, the recrystallized grain size is to foliation, and [100] forms a girdle parallel to foliation. This not unique throughout the hole, and shear zones having very crystallographic fabric was constructed from U-stage mea- small recrystallized grain sizes frequently cut deformed gab- surements of the optical directions and of the albite (010) or bros having larger recrystallized grain sizes (Cannat et al., this pericline twinning planes, using a computer program devised volume). These larger-grained recrystallized gabbros exhibit by Benn and Mainprice (1989). Contouring of diagrams was granulite-facies metamorphic assemblages, with no evidence performed using an unpublished program by D. Mainprice.

400 PLASTIC DEFORMATION AT AN OCEANIC SPREADING RIDGE

0.25 mm

0.04 mm

0.10 mm 0.05 mm Figure 2. Photomicrographs from Site 735. OL = olivine; CPX = clinopyroxene; PL = plagioclase; Hb = hornblende; OX = oxides. A. Foliation marked by tablet-shaped olivine and clinopyroxene crystals, Sample 118-735B-38R-4, 10-15 cm; this foliation is thought to have developed during viscous flow of the gabbro in the magmatic state; crossed nicols. B. Dynamic recrystallization of plagioclase in large polygonal neoblasts in Sample 118-735B-8OR-1, 11-17 cm. (010) albite twins in relict plagioclase porphyroclasts are parallel to the foliation (S) of the sample; crossed nicols. C. Dynamic recrystallization of olivine in large polygonal neoblasts in Sample 118-735B-80R-1, 11-17 cm. Subgrain boundaries in olivine porphyroclast are produced by (100) dislocation walls; crossed nicols. D. Dynamically recrystallized olivine, clinopyroxene, plagioclase, Fe-Ti oxide, and brown hornblende in a polymineral agregate in Sample 118-735B-80R-1, 11-17 cm. Note the poikilitic hornblende crystal; plane light. E. Large dislocation-free olivine neoblasts in Sample 118-735B-80R-1, 11-17 cm. The sample has been oxidized. The neighboring olivine grains, with a high density of dislocations, appear darker; plane light. F. Dynamic recrystallization of plagioclase in small, irregularly shaped neoblasts in Sample 118-735B-87R-5, 44-47 cm. (010) albite twins in dilacerated relict porphyroclasts are parallel to the foliation (S) of the sample. Most grains are slightly elongated in this foliation and exhibit wavy extinction and subgrain boundaries; crossed nicols.

401 M. CANNAT

0.10 mm 0.25 mm Figure 3. Photomicrographs from Site 735. A. Olivine and plagioclase grains elongated in the foliation (S) of Sample 118-735B-87R-5, 44-47 cm. Subgrain boundaries in olivine are produced by (100) dislocation walls. The plagioclase is recrystallized in small irregular neoblasts, and wavy extinctions are frequent; crossed nicols. B. Boudinaged plagioclase porphyroclast in finely recrystallized plagioclase matrix, Sample 118-735B-8D-1, 17-20 cm. The crack is subperpendicular to the stretching lineation (L) of the sample, and filled by green-brown hornblende and Fe-Ti oxide; crossed nicols. C. Fractured olivine crystal in Sample 118-735B-80R-5, 46-54 cm. The elongation of the hornblende grains in the sheared fracture indicates a dextral sense of shear in the photograph, corresponding to a normal offset in the sample. The olivine substructure is polygonized, with (100) and (001) dislocation walls (see Fig. 3D); crossed nicols. D. Detail of the polygonized dislocation substructure in the fractured olivine of Sample 118-735B-80R-5, 46-54 cm; oxidized sample. The (100) and (001) dislocation tilt walls isolate small square cells or subgrains; plane light. E. Coarse-grained Fe-Ti oxide-rich mylonite with clinopyroxene porphyroclast. Sample 118-735B-54R-1, 105-115 cm. See Figure 8A for reflected light. The recrystallized assemblage comprises oxides, plagioclase, clinopyroxene, olivine, orthopyroxene, and apatite. Magmatic apatite (AP) is enclosed in the relict clinopyroxene porphyroclast. Note also the relict magmatic zircon (Zr) in the upper left; plane light. F. Fine grained Fe-Ti oxide-rich mylonitic band in Sample 118-735B-77R-1, 106-116 cm. See Figure 8B for reflected light. The plagioclase and clinopyroxene porphyroclasts are rounded and unstrained; in the less oxide-rich portion of the sample (upper right), the obliquity between thin oxide-rich shear planes and the foliation marked elongated recrystallized plagioclase indicates a normal sense of shear; crossed nicols.

402 PLASTIC DEFORMATION AT AN OCEANIC SPREADING RIDGE

viscous flow. This should produce the observed [010] concen- trations perpendicular to the foliation (Figs. 5A and 5B), because (010) planes correspond to large crystal faces in both plagioclase and olivine. In samples showing a lineation, [100] in plagioclase may also tend to parallel the flow direction, because [100] often corresponds to the long axis of plagioclase crystals. The critical melt fraction separating this suspension- like flow behavior from solid-state flow is probably about 30 vol% (Van der Molen and Paterson, 1979). Below this critical value, the solidifying crystal mush behaved as a solid; con- tinuing flow produced mechanical twins, grain-boundary migration, and limited recrystallization in plagioclase, as well as the dislocations observed in many olivine grains.

SOLID-STATE PLASTIC DEFORMATION Deformed Gabbros Having Large B Recrystallized Grain Sizes Deformed gabbros having large recrystallized grain sizes are most frequent in the lower part of the hole (Cannat et al., this volume). Magmatic minerals (plagioclase, clinopyroxene, olivine, orthopyroxene, Fe-Ti oxides, and brown hornblende) have recrystallized into polygonal neoblasts that exhibit frequent triple junctions (Figs. 2B, 2C, and 2D). The grain size in monomineral olivine or plagioclase recrystallized aggregates is bimodal (Figs. 2B and 2C): small neoblasts (20 to 40 µm) are surrounded by larger grains (up to 0.2 mm in size). Most recrystallized plagioclase grains display mechanical twins (Fig. 2B). In olivine, the larger recrystallized grains are often free of dislocations (Fig. 2E), while most smaller neoblasts contain abundant free dislocations and dislocation walls. These small dislocation-rich grains appear to derive from the progressive misorientation of the subgrain boundaries formed by dislocation creep in the olivine porphyroclasts ("rotation recrystallization"; Poirier and Nicolas, 1975; Fig. 2C). The larger, often dislocation-free neoblasts are thought to result Figure 4. Sketches of deformed textures in the Site 735 gabbros. Scale from grain-boundary migration between dislocation-free and bar is 1 mm. Heavy contours: mafic porphyroclasts, mostly clinopy- dislocation-rich grains or subgrains ("migration recrystalliza- roxene; light contours: relict plagioclase; dots: recrystallized plagio- tion"; Poirier and Guillopé, 1979; Fig. 2E). The bimodal clase; little squares: recrystallized mafic minerals. A. Sample 118- grain-size distribution of the plagioclase neoblasts is attributed 735B-2D-2, 0-4 cm; weakly recrystallized, unfoliated. B. Sample to similar recrystallization mechanisms. 118-735B-3D-1, 46-49 cm; extensively recrystallized, mylonitic; In polymineral recrystallized aggregates, grain size is uni- closely spaced dots correspond to a band of finely recrystallized form and moderate ( 50 µm; Fig. 2D), probably because plagioclase and hornblende. recrystallized grains were prevented from growing by second- phase particles. Brown hornblende of probable magmatic origin (Stakes et al., this volume) is locally abundant. It has Because of the small size of the ODP samples, I was not able recrystallized in polygonal grains associated with clinopyrox- to cut the three orthogonal thin sections needed to ensure that ene or Fe-Ti oxide neoblasts, but it also formed poikiloblasts plagioclase crystallographic fabrics are truly representative enclosing neoblasts of other mineral phases (Fig. 2D). This (Kruhl, 1987; Benn and Allard, 1989). However, less than 25% suggests that this hornblende crystallized just after the end of of the plagioclase grains in the studied section exhibited no the deformation, from a fluid phase present along grain twins. I measured the optical directions of these untwinned boundaries. grains and compared them with the optical fabric of the The plagioclase and olivine crystallographic fabrics are twinned grains. Both grain populations exhibit similar optical scattered (Fig. 6). In plagioclase (Fig. 6A), there is a faint orientations, which suggests that the crystallographic fabric concentration of [010] perpendicular to the foliation. This diagrams of Figure 5A are reasonably representative. concentration is consistent with dislocation slip on (010), as A solid-state deformation large enough to produce this proposed by Ji et al. (1988) and by Ji and Mainprice (1988). It strong plagioclase crystallographic fabric presumably would is also compatible with slip on an irrational plane perpendic- also have resulted in an extensively recrystallized texture. It ular to the y optical indicatrix (Kruhl, 1989). Relict plagioclase also seems unlikely that the clinopyroxene and olivine crystals porphyroclasts exhibiting this orientation of [010] are elon- could have been mechanically flattened in the foliation with- gated in the foliation (Fig. 2B), but the recrystallized grains out breaking, recrystallizing, or developing a strong crystal- around them have more scattered crystallographic orienta- lographic preferred orientation. The observed foliation thus tions. Most (80%) of the plagioclase grains in the studied thin probably results largely from laminar flow of the incompletely sections are twinned. The optical fabric of untwinned grains crystallized magma (Benn and Allard, 1989). In this hypothe- compares well with that of twinned grains. Specifically, the sis, the crystals in suspension in the magma would tend to concentration of y (at about 30° of [010] for An50; Bum et al., arrange with their large crystal faces parallel to the plane of 1967) nearly perpendicular to the foliation is confirmed.

403 M. CANNAT

[010] [001] [100]

PL.

101meas. [100] [001] [010] B

OL.

80meas.

Figure 5. Viscous flow in the progressively solidifying magma. Plagioclase (An355 to An455) and olivine crystallographic fabrics from Sample 118-735B-38R-4, 10-15 cm. A. Plagioclase (PL) fabric (see text). B. Olivine (OL) fabric (see text). Lower hemisphere equal area projection. All diagrams in vertical plane; the arrow on the right side points upward. Line = trace of the foliation; the magmatic lineation is near downdip; black square = best axis of the distribution. Contours: 1;2;3;4% per 1% area.

[010] [001] [100]

PL.

[100] B

OL.

porph.+ ^ neobl.

[100]

OL. porph.

18meas.

Figure 6. Deformed gabbro having large recrystallized grains. Plagioclase (An50) and olivine crystallographic fabric from Sample 118-735B-80R-1, 11-17 cm. A. Plagioclase (PL) fabric (see text). B. Fabric of olivine (OL) porphyroclasts and neoblasts. C. Fabric of olivine porphyroclasts alone (see text). Lower hemisphere, equal area projection. All diagrams in vertical plane, arrow pointing upward. Line = trace of the foliation. The stretching lineation is downdip. Black square = best axis of the distribution. Contours are 1;2;3;4;8;16% per 1% area.

The olivine porphyroclasts and some neoblasts exhibit reorient the crystallographic fabric. This may result from the well-defined subgrain boundaries (Fig. 2C), as a result of the easy activation of the recrystallization mechanisms: as soon piling up of dislocations in walls perpendicular to [100] (ob- as some dislocations piled up in a grain, recrystallization served in oxidized samples; technique of Kohlstedt et al., occurred, producing new grains having an orientation similar 1976). This suggests that dislocation slip occurred along [100], to that of the original crystal. This process may have produced the common slip direction in olivine deformed at high temper- the observed crystallographic fabric (Figs. 6B and 6C), even ature (see Nicolas and Poirier, 1976). But the [100] axes in the after a large finite strain. fabric of the neoblasts and porphyroclasts (Fig. 6B) lie at a Easy activation of recrystallization mechanisms indicates high angle from the foliation and lineation of the sample. The high temperatures of deformation. This is consistent with the fabric diagrams in Figure 6C, including only the porphyro- granulite facies metamorphic assemblages. The presence of a clasts, are similar. This suggests that the strain accommodated water-rich fluid phase along the grain boundaries during by dislocation creep was never large enough in a given grain to deformation, suggested by the poikilitic brown hornblende

404 PLASTIC DEFORMATION AT AN OCEANIC SPREADING RIDGE crystals (Fig. 2D), may further enhance grain-boundary mi- preferentially near perpendicular to the lineation. a (at 18° of gration. Based on hornblende compositions (Stakes et al., this [100] for An45) is near to the lineation. This fabric is compat- volume), this fluid phase is thought to be magmatic in origin. ible with dislocation slip on (010) (Ji et al., 1988; Ji and Mainprice, 1988), or on an irrational plane perpendicular to Deformed Gabbros Having Small the y optical indicatrix (Kruhl, 1989). Note that this plagio- Recrystallized Grain Sizes clase crystallographic fabric does not account for the strong Deformed gabbros having small recrystallized grain sizes anisotropy of seismic velocities measured in some Site 735 are frequent in shear zones throughout the hole (Cannat et al., deformed gabbros (Shipboard Scientific Party, 1989). These this volume). These shear zones often cut the deformed measurements indicate that the slowest direction of seismic- gabbros having larger recrystallized grains. They developed wave propagation is often perpendicular to foliation. In pla- over a range of metamorphic conditions that covered the gioclase having a strong magmatic or solid-state fabric, the stability fields of upper to lower amphibolite-facies assem- perpendicular to the foliation coincides with the maximum blages (Stakes et al., this volume). The deformed samples I concentration of [010], which is thought to correspond with a have selected for this study exhibit upper amphibolite-facies fast direction of seismic-wave propagation in the feldspar metamorphic assemblages. Most magmatic minerals have crystals (Ji and Mainprice, 1988). The concentration of the y recrystallized in these samples (plagioclase, olivine, clinopy- optical indicatrix near the lineation (Fig. 7A) suggests that roxene, brown hornblende, and Fe-Ti oxides), but orthopy- [100] may have acted as the dominant slip direction. Possibly, roxene grains are fractured, and green to brown hornblende of slip along [100] was favored in my samples by a preexisting probable hydrothermal origin (Stakes et al., this volume) magmatic fabric, with a lineation defined by the long axis crystallized in the foliation. In shear zones developed in the ([100]) of the plagioclase crystals and parallel to the solid-state stability conditions of actinolitic hornblende, small recrystal- stretching lineation. lized plagioclase and oxide grains are also present, but the In the most extensively recrystallized samples, the shape mafic minerals behaved in a brittle fashion. and optical fabrics of plagioclase are still strong, and grains The recrystallized plagioclase grains are irregularly shaped exhibit abundant subgrain boundaries. This indicates that and 5 to 10 µm in size (Fig. 2F). These grains appear to derive dislocation creep and dynamic recrystallization remained the from the progressive misorientation of subgrains in the de- dominant flow mechanisms. But the relict plagioclase porphy- formed crystals ("rotation recrystallization"; Fig. 2F). The roclasts elongated in this fine-grained recrystallized matrix are recrystallized grain-size distribution is unimodal, and triple often fractured and boudinaged (Fig. 3B), even when their junctions are infrequent. This suggests that diffusion-con- crystallographic orientation favored dislocation slip (y perpen- trolled migration recrystallization (Poirier and Guillopé, 1979) dicular to foliation). This indicates that the recrystallized was not very active. The deformed plagioclase grains have a matrix was able to flow faster than the nonrecrystallized good shape fabric, parallel to the foliation and lineation of the porphyroclasts. This strain-softening effect probably resulted samples (Fig. 3A). These grains are rarely twinned, but exhibit from the continuous replacement through dynamic recrystal- pronounced undulatory extinction. Their preferred orientation lization of strain-hardened grains having abundant disloca- is strong (Fig. 7A). Because of the absence of twinned grains, tions by annealed, dislocation-free neoblasts. Cracks in the only the optical fabric could be measured, y (at 25° of [010] for broken plagioclase porphyroclasts are filled with green to An45; Burn et al., 1967) is preferentially perpendicular to the brown hydrothermal hornblende (Stakes et al., this volume) foliation, ß (at 50° of [001] for An45) tends to lie in the foliation, and by secondary Fe-Ti oxides or sulfides (Fig. 3B).

PL.

110meas. B

OL.

Figure 7. Deformed gabbros having small recrystallized grains. Plagioclase (OL) and olivine (OL) fabrics from Samples 118-735B-87R-5, 44-47 cm, and 118-735B-8D-1, 17-20 cm. A. Plagioclase (An4o to An45); optical fabric in Sample 118-735B-87R-5, 44-47 cm. B. Olivine fabric (porphyroclasts and neoblasts) in Sample 118-735B-87R-5, 44-47 cm. C. Olivine fabric (porphyroclasts and neoblasts) in Sample 118-735-8D-1, 17-20 cm. Projection conventions and symbols, same as Figure 6. Contours: 1;2;3;4;8% per 1% area.

405 M. CANNAT

Olivine porphyroclasts are elongated parallel to the linea- porphyroclasts and the mafic minerals (olivine and pyroxene). tion of the gabbro (Fig. 3A). They have closely spaced In addition, this effect was contemporaneous with circulation subgrain boundaries that correspond to (100) dislocation walls through the gabbros of a hydrothermal fluid crystallizing (observed in oxidized samples). Olivine is frequently recrys- brown to green hornblende. tallized into polygonal grains 20 to 100 µm in size. The dislocation substructure in these neoblasts is usually dense, Strain Localization: Deformation Processes but some grains are dislocation-free and probably result from in Site 735 Mylonites migration recrystallization. The crystallographic fabric of both porphyroclasts and neoblasts is strong (Figs. 7B and 7C). This Deformed Gabbros Having Large Recrystallized Grain Sizes fabric corresponds to dislocation slip parallel to [100] on the In deformed gabbros having large recrystallized grains, a (010) planes (Fig. 7B), or to a combination of the (010) and substantial reduction in grain size was observed in polymin- (001) planes (Fig. 7C). The obliquity between [100] and the eral recrystallized aggregates (Fig. 2D, compared to Figs. 2B stretching lineation of the gabbro indicates a strong simple and 2C). But mylonitic bands (i.e., narrow bands of high strain shear component (Bouchez et al., 1983) that corresponds to a with reduced grain size; Bell and Etheridge, 1973) developed normal offset in the two studied samples (Figs. 7B and 7C). only in intervals containing more than about 10% oxides (Fig. The slip system identified in the olivine of the Site 735 samples 3E). The modal proportions of oxides have been measured in has been activated experimentally at high, near-solidus tem- four mylonite samples using the "Visilog" image-processing peratures (Raleigh and Kirby, 1970; Carter and Ave Lalle- software (NOESIS, 1988), in microphotographs taken in re- mant, 1970; Phakey et al., 1972; Durham et al., 1977; Jaoul et flected light (see Fig. 8). Minor quantities of recrystallized al., 1979). During experimental deformation of olivine at lower sulfides, present in all samples, are included in the oxide temperatures, more compatible with the hornblende-bearing modal content. In these oxide-rich mylonites, ilmenite and metamorphic assemblage observed in my samples, the domi- magnetite recrystallized in euhedral grains 5 to 40 µm in size nant slip direction was [001] (Raleigh and Kirby, 1970; Carter and formed discontinuous streaks, with no preferred elonga- and Ave Lallemant, 1970; Phakey et al., 1972). Note, how- tion (Figs. 3E and 8A) along grain boundaries of the other ever, that these experiments were performed at differential recrystallized minerals and of the rounded porphyroclasts. stresses and strain rates probably much higher than those that Recrystallized silicates are polygonal, equant, 10 to 40 µm in prevailed during the deformation of the Site 735 gabbros. For example, Phakey et al. (1972) conducted their experiments at differential stresses between 4 and 10 kb and strain rates about A 10"5s-'. In samples containing the most extensively recrystallized plagioclase (Fig. 3B), the strain-softening effect that led to boudinage of the plagioclase porphyroclasts also caused brittle failure of the clinopyroxene, orthopyroxene, and olivine grains. These fractures are similarly filled with hornblende, and sometimes with secondary Fe-Ti oxides. They are either perpendicular to the stretching lineation, as in the case of the plagioclase porphyroclast shown in Figure 3B, or oblique on the foliation and sheared with a normal offset (Fig. 3C). Near the fractures, the olivine has a tightly polygonized substructure (Fig. 3C) caused by a dense array of (100) and (001) dislocation tilt walls (observed in oxidized samples; Fig. 3D). I propose that this strain-hardened substructure resulted either from the increased strain rate imposed on olivine by the surrounding recrystallized plagioclase, or from a progressive temperature decrease during the deformation. At this higher B strain rate, or lower temperature (still within the temperature range of hornblende stability), dynamic recrystallization in olivine could not compete efficiently with the hardening effect of the piling up of dislocations. As a result, the olivine crystals became harder and harder to deform plastically and eventually broke. In summary, the dominant flow mechanisms in deformed gabbros having small recrystallized grain sizes appear to have been dislocation creep and dynamic recrystallization of plagioclase. In olivine, dislocation creep along high-temperature slip systems produced strong crystallographic fabrics. Dy- namic recrystallization-occurred through both rotation and migration processes. But this latter process may have been less active here than in the deformed gabbros having large 0.02mm recrystallized grain sizes. This is consistent with the metamorphic assemblages, which indicate amphibolite, rather than Figure 8. Fe-Ti oxide-rich mylonites in reflected light. A. Coarse- grained mylonite, oxide modal content: 10.9%; Sample 118-735B- granulite, facies temperatures of deformation. In those inter- 54R-1, 105-115 cm; see Figure 3E for transmitted light. B. fine-grained vals exhibiting extensive recrystallization of plagioclase, a mylonite, oxide modal content: 12.6%; Sample 118-735B-77R-1, 106- strain-softening effect was observed. This effect is thought to 116 cm; see Figure 3C for transmitted light. Oxide modal contents have been controlled by dynamic recrystallization of plagio- have been measured with "Visilog" image-processing software clase, and it led to brittle failure of both the plagioclase (NOESIS, 1988).

406 PLASTIC DEFORMATION AT AN OCEANIC SPREADING RIDGE size, and exhibit no crystallographic fabric. When present, accommodated grain-boundary sliding, as suggested for the olivine is dislocation-free. This indicates either that, disloca- coarser-grained mylonites. tion creep was not an active flow mechanism, or that the dislocation substructure was efficiently annealed by diffusion- Origin of Oxides in the Mylonites controlled migration recrystallization, as in the less-deformed The origin of oxides in the oxide-rich mylonites at Site 735 surrounding gabbros. Relict porphyroclasts in these mylonites was discussed among the Leg 118 shipboard scientists (Ship- are rounded, show no preferred elongation, and are neither board Scientific Party, 1989). Was the oxide concentration in kinked nor fractured. They appear to have been passively these mylonites primary, or did it result from channeling in rotated in the recrystallized matrix. The streaks of recrystal- shear zones of hydrothermal fluids rich in dissolved oxides? lized oxides are not connected: they do not continuously There is evidence for remobilization of oxides by the surround the grain boundaries of the other minerals (Fig. 8A). water-rich hydrothermal fluids circulating in the gabbros be- Deformation thus is unlikely to have been controlled only by fore the end of deformation: oxides may be associated with plastic flow in the ilmenite and magnetite crystals. But be- green-brown hornblende in synkinematic cracks perpendicu- cause only intervals containing more than 10% oxides are lar to stretching lineation (Fig. 3B). However, the oxide-rich mylonitized, the oxides are thought to have played an impor- mylonites I have studied also contain abundant calcium-poor tant role in the activation of the strain-softening mechanisms, pyroxene (orthopyroxene or inverted pigeonite), locally abun- leading to the formation of mylonites. To account for the dant apatite, and scattered zircon (Fig. 3E). The apatite in absence of foliation (Fig. 8A), the equant shape of the grains, these mylonites demonstrably recrystallized from magmatic the lack of crystallographic fabric and of a dislocation sub- apatite enclosed in the relict clinopyroxene porphyroclasts structure (in olivine), and the passive rotation of the porphy- (Fig. 3E). These minerals are typical of undeformed oxide-rich roclasts, I propose that flow in these mylonites was controlled intervals that exhibit preserved magmatic textures (Shipboard by mutually accommodating grain-boundary sliding and diffu- Scientific Party, 1989). Modal proportions of oxides in the sion creep, as defined by Raj and Ashby (1971), Boullier and mylonites (10% to 18%) are well within the composition range Gueguen (1975), and Gifkins (1976). According to these au- of these oxide-rich undeformed gabbros (5% to 25%; Ship- thors, reduced grain size, high temperature, and moderate board Scientific Party, 1989), and relative proportions of deviatoric stresses favor these two deformation mechanisms. ilmenite and magnetite are similar (1:2 to 2:1). I therefore Diffusion creep may have been further enhanced by (1) the propose that most oxides present in the mylonites I have increased diffusivity of the grain boundaries between unlike studied derive from magmatic oxides through dynamic recrys- phases; (2) the positive dependence of the diffusivity in oxides tallization. on the oxygen partial pressure (see Poirier, 1985), which may have been high in the studied samples because magnetite is CONCLUSIONS abundant (the estimated ilmenite/magnetite ratio ranges from Microstructural analysis of Site 735 gabbros suggests that 1:2 to 2:1 in different samples and from place to place in a some gabbros have been deformed in the crystal mush stage, given sample); and (3) the dispersal of an aqueous fluid phase before they had completely crystallized. This magmatic defor- along grain boundaries during the deformation, suggested by mation produced a foliation and a strong plagioclase crystal- the local presence of poikilitic brown hornblende (Fig. 2D). lographic fabric. Later, solid-state deformation occurred at progressively lower temperatures and increased availability of Deformed Gabbros Having Small Recrystallized Grain Sizes hydrothermal water, probably as the gabbros spread away In deformed gabbros having small recrystallized grain from the ridge axis (Cannat et al., this volume). The micro- sizes, dynamic recrystallization of plagioclase in monomineral structural study shows a sharp change of deformation pro- aggregates may have led to significant strain softening (see cesses in the gabbros as temperature decreased from the above paragraph). The resulting mylonites (Fig. 3B) are char- stability conditions of granulite-facies metamorphic assem- acterized by a strong shape and crystallographic fabric of blages to the stability conditions of amphibolite-facies meta- plagioclase and by the development of sigmoidal tails around morphic assemblages. Temperature-dependent diffusion pro- most relict porphyroclasts (Fig. 4B). cesses may have controlled the deformation of all minerals in But the most spectacular mylonites developed in polymin- granulite-facies metamorphic conditions. Disorganized plagio- eral recrystallized aggregates containing abundant Fe-Ti ox- clase and olivine fabric patterns characterize this granulite ides and/or synkinematic hornblende. The Fe-Ti oxide-rich facies deformation. In contrast, dislocation slip in plagioclase mylonites are almost completely opaque in transmitted light appears to have been the leading flow mechanism in gabbros (Fig. 3F). This suggests a very high oxide content, but deformed in amphibolite-facies metamorphic conditions and reflected light observations (Fig. 8B) revealed that the modal produced strong plagioclase and olivine crystallographic fab- proportion of oxides in these mylonites is actually similar to rics. This change of deformation processes correlates with a that of the coarser-grained mylonites produced in granulite marked size decrease of the recrystallized plagioclase grains. facies metamorphic conditions (Fig. 8A). Grain size is about This reduction in recrystallized grain size is thought to result 10 times smaller (Fig. 8), but the texture is about the same in from an increase of the deviatoric stress, and thus from an the two types of mylonites: discontinuous streaks of recrys- increase of the yield strength of the gabbros. tallized oxides (Figs. 8A and 8B), no foliation, and rounded During the deformation in granulite-facies metamorphic and unstrained relict porphyroclasts (Fig. 3F). Apart from the conditions, mylonites formed only in intervals containing oxides, the very fine-grained recrystallized matrix is com- more than 10% oxides. Strain-softening in these intervals may posed of plagioclase and hornblende. Because of the opacity have been caused by the activation of diffusion creep-accom- of the rock in transmitted light, the degree of crystallographic modated grain-boundary sliding. Most oxides in these mylo- preferred orientation of these minerals is impossible to assess nites are interpreted here as magmatic in origin. This suggests with the optical microscope. If this preferred orientation is that strain localization leading to the formation of ductile strong, a possible flow mechanism may have been dislocation faults in the deepest and hottest parts of the oceanic litho- slip-accommodated grain-boundary sliding, as proposed by sphere may be influenced by the magmatic stratigraphy. Etheridge and Wilkie (1979). Otherwise, I propose that this During the deformation in amphibolite-facies metamorphic deformation may have been controlled by diffusion creep- conditions, dynamic recrystallization of plagioclase in very

407 M. CANNAT small neoblasts may have had a strain-softening effect, leading Jaoul, O., Gueguen, Y., Michaut, M., and Ricoult, D., 1979. A to formation of mylonitic bands and to brittle failure of the technique for decorating dislocations in forsterite. Phys. Chem. larger relict porphyroclasts (plagioclase, olivine, and pyrox- Mineral., 5:15-20. ene). Note that this brittle failure occurred at temperatures Ji, S., and Mainprice, D., 1988. Natural deformation fabrics of plagioclase: implications for slip systems and seismic anisotropy. still within the stability range of brown to green hornblende. Tectonophysics, 147:145-163. Ji, S., Mainprice, D., and Boudier, F., 1988. Sense of shear in high temperature movement zones from the fabric asymmetry of pla- ACKNOWLEDGMENTS gioclase feldspars. J. Struct. Geol., 10:73-81. Kirby, S. H., 1985. Rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of Many thanks to the students and staff of the Laboratoire de strain along shear zones. Tectonophysics, 119:1-27. Tectonophysique, USTL, Montpellier (France), and espe- Kohlstedt, D. L., Goetze, C, Durham, W. B., and Vander Sande, J., cially to D. Mainprice and K. Benn, who allowed me to use 1976. A new technique for decorating dislocations in olivine. their interactive program for measuring plagioclase fabrics, Science, 191:1045-1046. Kruhl, J. H., 1987. Preferred lattice orientations of plagioclase from and to A. Nicolas, for his helpful comments about the manu- amphibolite and greenschist facies rocks near the Insubric Line script. I also thank M. Boeuf, a student at UBO, Brest, for his (Western Alps). Tectonophysics, 135:233-242. help with the image-processing software, and J-L. Travers , 1989. Natural deformation of plagioclase: implications for (UBO, Brest), for his help with the graphics. This work was slip system and seismic anisotropy—a discussion on measuring financially supported by ODP-France. and interpreting plagioclase preferred orientations. Tectonophys- ics, 166:345-350. REFERENCES Mercier, J-C, Anderson, D. A., and Carter, N. L., 1977. Stress in the Bell, T. H., and Etheridge, M. A., 1973. Microstructure of mylonites lithosphere: inferences from steady-state flow of rocks. Pure Appl. and their descriptive terminology. Lithos, 6:337-348. Geophys., 115:199-226. Benn, K., and Allard, B., 1989. Preferred mineral orientations related Nicolas, A., and Poirier, J-P., 1976. Crystalline Plasticity and Solid to magmatic flow in ophiolite layered gabbros.J. Petrol.,30:925- State Flow in Metamorphic Rocks: London (Wiley Interscience). 946. NOESIS, 1988. "Visilog," a computer vision software: Versailles, Benn, K., and Mainprice, D., 1989. An interactive program for France. determination of plagioclase crystal axes orientations from Phakey, P., Dollinger, G., and Christie, J. M., 1972. Transmission U-stage measurements: an aid for petrofabric studies. Computers electron microscopy of experimentally deformed olivine crystals. and Geosciences, 15:1127-1142. In Heard, H. C. (Ed.), Flow and Fracture of Rocks. Geophys. Bird, J. E., Mukherjee, A. K., and Dorn, J. F., 1969. Correlations Monogr. Ser., 16:117-138. between high temperature creep behavior and structure. In Bran- Poirier, J-P., 1985. Creep of Crystals. High Temperature Deformation don, D. G., and Rosen, R. (Eds.), Quantitative Relation Between Processes in Metals, Ceramics and Minerals: Cambridge (Cam- Properties and Microstructure: Haifa, Israel, 255-342. bridge Earth Sci. Ser.). Bouchez, J-L., Lister, G. S., and Nicolas, A., 1983. Fabric asymme- Poirier, J-P., and Nicolas, A., 1975. Deformation-induced recrystal- try and shear sense in movement zones. Geol. Runds., 72:401— lization by progressive misorientation of subgrain-boundaries, 419. with special reference to mantle peridotites. J. Geol., 83:707-720. Boullier, A-M., and Gueguen, Y., 1975. SP-mylonites: origin of some Poirier, J-P., and Guillopé, M., 1979. Deformation-induced recrystal- mylonites by superplastic flow. Contrib. Mineral. Petrol., 50:93- lization of minerals. Bull. Mineral., 102:67-74. 104. Raj, R., and Ashby, M. F., 1971. On grain boundary sliding and Burri, C, Parker, R. L., and Wenk, E., 1967. Die optische Orien- diffusional creep. Metall. Trans., 2:1113-1127. tierung der Plagioklase: Basel (Birkhaüser). Raleigh, C. B., and Kirby, S. H., 1970. Creep in the upper mantle. Carter, N. L., and Ave Lallemant, H. G., 1970. High temperature Mineral. Soc. Am. Spec. Pap., 3:113-121. flow of dunite and peridotite. Geol. Soc. Am. Bull., 81:2181-2202. Shipboard Scientific Party, 1989. Site 735. In Robinson, P. T., and Durham, W. B., Goetze, C, and Blake, B., 1977. Plastic flow of Von Herzen, R., et al., Proc. ODP, Init. Repts., 118: College oriented single crystals of olivine. 2. Observations and interpreta- Station, TX (Ocean Drilling Program), 89-212. tions of the dislocation structures. J. Geophys. Res., 82:5755- Twiss, R. J., 1977. Theory and applicability of a recrystallized grain 5770. size paleopiezometer. Pure Appl. Geophys., 115:227-244. Etheridge, M. A., and Wilkie, J. C, 1979. Grain size reduction, grain Van der Molen, I., and Paterson, M. S., 1979. Experimental defor- boundary sliding and the flow strength of mylonites. Tectonophys- mation of partially melted granite. Contrib. Mineral. Petrol., ics, 58:159-178. 70:299-318. Gandais, M., and Willaime, C, 1984. Mechanical properties of feldspars. In Brown, W. L. (Ed.), Feldspars and Feldspathoids: NATO ASI Series, C-137, Reidel Dordrecht, 207-246. Date of initial receipt: 11 July 1989 Gifkins, R. C, 1976. Grain boundary sliding and its accommodation Date of acceptance: 14 March 1990 during creep and superplasticity. Metall. Trans., 7:1225-1232. Ms 118B-134

408